Holograms are optical elements which are recordings of a pre-existing scene or image. In the recording of a hologram, scattered light from an object is incident onto a recording medium where it interferes with a reference beam such that the interference pattern is recorded in the recording medium (e.g. a photographic plate). The result of recording the image in this manner is that if the recorded interference pattern is then illuminated by the reference beam, the original light field is reconstructed, thus giving the appearance of the presence of the original object. As well as being able to replay holograms by illuminating the rear side with a reference beam, holograms may also be constructed by coating the rear side with a reflective material and then viewing the hologram from the front side.
If the interference pattern is recorded as relief in a photosensitive medium such as a photoresist then it may be reproduced by producing a metal negative copy of this relief and then using that metal negative to emboss the relief into other media such as softened plastics by a hot stamping technique. Typically, such a metal relief would be produced by the method of nickel electroforming.
The technique of hot stamping of diffractive surfaces is often used in the security industry as an anti-counterfeiting measure. This approach is commonly used to demonstrate the authenticity of items such as credit cards or high value branded items such as mobile phone batteries or original manufacturer ink cartridges.
In many cases, the diffractive master may not have been formed as a hologram. However, in all cases the common feature is a structure on the element that is sufficiently small to cause the diffraction of light which is incident upon it.
In the case of hot stamping of nickel formed masters, each diffractive element produced is identical. In some cases, a degree of personalisation or uniqueness is introduced by evaporating the reflective rear layer through a mask so as not to cover the whole of the embossed image. However, this can only produce as many different elements as there are evaporation masks.
It would be highly desirable in the industry to be able to produce such elements digitally so that unique images or personal data could be recorded with the inherent protection of holographic or diffractive structures.